It has been known to use a valve operation characteristics changing device in an outboard motor where the valve operation characteristics changing device is adapted to variably control operation characteristics of intake and exhaust valves for controlling supply of air-fuel mixture and discharge of combustion gas, respectively, depending on an operational condition of an engine of the outboard motor, to whereby achieve low fuel consumption and high output over a wide operation range (see Japanese Patent Application Laid-Open (kokai) No. 2000-186516).
On the other hand, in order to optimally control the air-fuel ratio to achieve higher combustion efficiency, it is desired to provide an exhaust gas property sensor at an appropriate position in an exhaust passage (see Japanese Patent Application Laid-Open (kokai) No. 7-301135).
However, in the conventional structure disclosed in the above-mentioned JPA Laid-Open No. 2000-186516, a selector valve for controlling the valve operation characteristics changing device is accommodated in a head cover for covering a valve gear mechanism, which results in a larger head cover. This leads to a larger engine cover and increases an amount of overhang in a tilt-up position, which is unfavorable in outboard motors where the engine cover having a smaller outer profile is preferred. Further, the exhaust gas property sensor protrudes from the component parts constituting the exhaust passage, and this can also hinder achieving a smaller engine cover.